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• Hydrogen partial pressure or H2/HC recycle ratio.

• Quality of the feed.

• Coke content of the spent catalyst at last reactor outlet.

The above are independent variables: each of them can be fixed by the operator - within the operating range of the equipment - independently from the others.

For one set of independent variables, for same feed characteristics, there is only performance of the unit i.e. one set of values for:

• Product yields.

• Product quality.

• Catalyst stability (coke make).

In this chapter we examine the effect on the unit performance of each independent variable taken separately.

6.3.4.2 Pressure

Hydrogen partial pressure is the basic variable because of its inherent effect on reaction rates.

But for the ease of understanding total reactor pressure can be used. Reactor pressure is most accurately defined as the average catalyst pressure. Due to catalyst distribution in the reactors, it is usually close to the last reactor inlet pressure.

All the hydrogen producing reactions i.e. dehydrogenation, dehydrocyclisation are enhanced by low pressure.

The lower the pressure the higher the yields of both reformate and hydrogen for a given octane number. This is the reason for minimizing unit pressure drop and operating at the lowest practical pressure. Low pressure however increases the coke make.

Operators action on pressure is limited:

• Operating pressure rise is limited by equipment design pressure.

• Operating pressure lowering is limited by recycle compressor design power and

6.3.4.3 Temperature

In the Aromizing ® unit the coke amount on the catalyst is maintained at a constant low level through the continuous regeneration. Consequently a temperature adjustment is required only:

• To process a different feed quantity.

• To process a different feed quality.

• To balance a temporary loss of activity due to a temporary poisoning.

• To balance catalyst ageing which occurs slowly over several years.

An increase of the reactor inlet temperature results in :

• An increased conversion of the non aromatic compounds of the feed mainly the paraffins. But since the hydrocracking reaction is more favored than the cyclization of paraffins, the end result is:

- An increased octane but a decrease in aromizate yield.

- An increase of the coke deposit which is compensated by a catalyst circulation increase to maintain at same level the coke content of the catalyst.

6.3.4.4 Space velocity

The space velocity is the amount of liquid feed, expressed in weight (or in volume) which is processed in one hour, divided by the amount of catalyst contained in reactors, expressed in weight (or in volume). Weight (volume) of feed and catalyst must be expressed with the same unit.

Weight Hourly Space Velocity: ^WHSV= _Weight ^Weight _of^of_catalyst ^feed(per _{in reactors}^hour)

Liquid Hourly Space Velocity: ^LHSV=Volume_Volume^of_of^feed_catalyst ^{at 15°}_in ^C_reactors^{(per hour)} The inverse of the liquid hourly space velocity i.e. (LHSV)-1 is linked with the residence time of the feed in the reactor. The space velocity then affects directly the kinetics of the reforming reactions.

A decrease in the space velocity means an increased residence time, hence a higher severity which results in increased octane, lower aromizate yield, higher coke deposit.

When changing feed rate, an important recommendation derives from the above:

• Always decrease reactor inlet temperature first and decrease feed flow rate afterwards.

• Always increase feed flow rate first and increase temperature afterwards.

6.3.4.5 Hydrogen to hydrocarbon ratio and hydrogen partial pressure

The H2/HC ratio is the ratio of pure hydrogen in the recycle gas (mole/hour) to the feed flow rate (mole/hour). flexibility in the total pressure, hydrogen partial pressure is mainly adjusted through recycle flow.

Recycle hydrogen is necessary in the reformer operation for purposes of catalyst stability. It has the effect of sweeping the reaction products and condensable materials from the catalyst and supplying the catalyst with readily available hydrogen. An increase in H2/HC ratio will move the naphtha through the reactors at a faster rate and supply a greater heat sink for the endothermic heat of reaction. The end result is an increased stability.

A lower H2/HC ratio decreases the hydrogen partial pressure and increases coke formation.

Within the typical operating range, the H2/HC ratio has little influence on product quality or yields. It is not a variable that the operator typically adjusts, it is set by design based on an economic balance between equipment sizing i.e. recycle compressors, fired heaters and the regeneration unit.

Moreover, for a given unit, the amount of recycle is limited by the recycle compressor characteristics (power, suction flow).

6.3.4.6 Feed quality A Distillation range

Light fractions have a poor naphthenic and aromatic content and consequently a high C6 paraffinic content. Cyclization of C6 paraffins to aromatics is more difficult than cyclization of C7 or C8 paraffins, as discussed in “Kinetics analysis” (point C).

Hence, for a required octane number, the lighter the feed the higher the required severity or, conversely, at constant severity, low initial boiling point results in lower aromatic and hydrogen yields.

In recent years, the restriction of benzene content in gasoline has resulted in selecting feed with IBP above 82°C to eliminate cyclohexane.

Heavy fractions have a high naphthenic and aromatic hydrocarbons content, thus they need a lower severity to obtain good yields. But these fractions contain also polycyclic compounds which produce a high coke deposit on the catalyst. High final boiling point of the feed is favorable up to a certain level, detrimental above. An end boiling point above 180°C is

B Chemical composition

The detailed chemical composition of the feed is determined by gas chromatography analysis.

This analysis is necessary to predict the aromatics and hydrogen production as well as the severity of the operation.

Even if not sufficient for a complete prediction, an index of characterization of the feedstocks related to the actual and potential aromatics content of the feed, proved very useful. N + 2A has long been used (N and A volume % of naphthenes and aromatics in the feed). AXENS now uses 0.85 N + A which is found to be more representative.

The higher this index, the lower the severity of operation to meet the same product specifications. The lower this index (i.e. the higher the paraffins content), the higher the severity of operation to meet the same product specifications as the dehydrocyclization of paraffins becomes important.

Note that cracked naphthas have a ratio naphthenes C6 nucleus / naphthenes C5 nucleus much lower that SR naphthas.

*: Starting with 9 carbon atoms analysis cannot allow to segregate NNC5 from NNC6.

Remember that aromatization of NNC5 requires first an isomerisation in NNC6. If this process is rather good for C7+ naphthenes it is only ∼60% for the methylcyclopentane while the cyclohexane is completely converted in benzene.

C Impurities in the feed

The catalyst activity can be reduced, either temporarily or permanently by poisons contained in the feed (refer to paragraph “Catalyst contaminants”). Table 5 specifies the maximum allowable amount of each contaminant. Methods of analyses are given in chapter “Analytical control".

D Summary

Table 4 summarizes the theoretical effect on the unit performance of each independent process variable taken separately.

TABLE 4